1. Field of the Invention (Technical Field)
The present invention relates to orbital propulsion and power generation systems, and, more particularly, to a method and apparatus of using a spinning (fast-rotating) conducting tether to produce an electrodynamic propulsion force to change the orbit of a satellite through the interaction between the electric current in the tether and the external magnetic field or to produce electric power using electromotive force in the tether.
2. Background Art
Space tethers have attracted a lot of attention in the past 40 years. Many researchers have contributed to the theory of tether behavior in orbit. The theory has been applied and proved in a number of flights involving tethers attached to spacecraft.
In 1966, Gemini 11 and 12 manned spacecraft were attached with a tether to a rocket stage and demonstrated libration and rotation modes of tethered motion.
In 1992, TSS-1, the Shuttle-based Tethered Satellite System, including a 550-kg satellite and a 20-km electrically conductive tether, was partially deployed from the Shuttle orbiting at an altitude of 296 km. The measurement of the voltage-current profiles shed new light on electric behavior of conducting tethers in orbit.
In 1993, SEDS-I, the Small Expendable Deployment System, including a 26-kg mini-satellite on a 20-km non-conductive tether, was successfully deployed downward from a Delta rocket second stage. SEDS-I was a flight experiment to test the deployment of a long tether by means of a light and simple deployment mechanism and the deorbit and reentry of the mini-satellite after the release of the tether from the Delta stage. It was the longest structure (20 km) ever deployed in orbit.
Also in 1993, PMG, the Plasma Motor Generator, including a 500-m-long electrodynamic tether, was deployed from the Delta second stage with the primary goal of testing power generation and thrust by means of an electrodynamic tether. This mission was the first example of a propulsion system for space transportation that did not utilize any propellant, but rather achieved propulsion by converting orbital energy into electrical energy (deorbit) or electrical energy into orbital energy (orbit boosting).
In 1994, SEDS-II, the Small Expendable Deployment System (second flight), with the same equipment of SEDS-I, was utilized for a longer and more ambitious mission. The system was stabilized along the local vertical at the end of deployment and kept attached to the Delta stage to study the acceleration environment and, during the extended mission phase, the survivability of a thin tether to micrometeoroid impacts. During the extended mission phase, SEDS-II also provided important data on the micrometeoroid risk as the tether was cut at the 7-km point three days after the completion of the one-day primary mission.
In 1996, TSS-1R, a reflight of TSS-1 was attempted. The mission was terminated before due time by an electrical arc that severed the tether just before the end of deployment. Nevertheless, it was an important mission for tethered satellites because it showed that the electrodynamic tethers were more efficient that theoretically predicted, providing valuable data on electric performance of the system.
In 1996, TiPS, the Tether Physics and Survivability Experiment, including a 4-km-long passive tethered system for the investigation of the long-term survivability of tethers in the space environment, was successfully started. This system proved that a sufficiently fat tether can survive for a very long time the harsh space environment, and also provided valuable data on the long-term passive internal damping of tether librations.
In 1996, the Advanced Tether Experiment (ATEx) began deployment in orbit. About 18 minutes into deployment, at a deployed length of only 22 meters, the tether went slack, bent, and triggered several tether departure angle optical sensors. This led to the tether experiment being automatically ejected, to protect the host vehicle. The slackness occurred just after sunrise any may have resulted from a thermal transient on the thin polyethylene tape tether.
In 2002, ProSEDS, the Propulsive Small Expendable Deployer System, will deploy 10 km of Dyneema tether followed by 5 km of bare wire from a Delta-II stage to test the electrodynamic propulsion capabilities of the tether.
“Tethers in Space Handbook,” Second Edition, NASA Office of Space Flight, NASA Headquarters, Washington, D.C., 1989, edited by P. A. Penzo and P. W. Ammann, provides summaries of various applications and features of space tethers, including methods to change orbital elements with electrodynamic tether propulsion and methods to control the attitude dynamics of such tethers. The basic concept is to vary the electric current in the tether based on the estimate of the tether state obtained from measurements of certain tether system parameters. It is noted on p. 8 of the Handbook that the electrodynamic tether of the Plasma Motor Generator (PMG) deployed from the Shuttle orbiter to conduct plasma physics experiments may be centrifugally stabilized by rotation at 15 revolutions per orbit, while the operation in this mode is not described.
The following patents cover certain details of electrodynamic tether usage.
U.S. Pat. No. 6,116,544, entitled “Electrodynamic Tether and Method of Use,” issued Sep. 12, 2000, to Forward et al., describes electrodynamic tethers for deorbiting out-of-service satellites. In one embodiment, the tether rotates about the center of mass “to centrifugally produce tension force in the tether”, which recites the concept of centrifugal stabilization of an electrodynamic tether, described earlier in the “Tethers in Space Handbook”, NASA, 1986. This patent does not disclose design, operational, and performance advantages and methods of use of spinning (fast-rotating) electrodynamic tethers. The '544 Patent claims that the optimal mode of tether operation is non-spinning, with the tether hanging at a certain fixed angle relative to the local vertical.
U.S. Pat. No. 6,260,807, entitled “Failure Resistant Multiline Tether,” issued Jul. 17, 2001, to Hoyt et al., discusses various multistrand tethers to improve strength and stability.
U.S. Pat. No. 4,923,151, entitled “Tether Power Generator for Earth Orbiting Satellites,” issued Mar. 1, 1988 to Roberts et al., discloses use of an electrodynamic tether as a power generator for earth orbiting satellites.
U.S. Pat. No. 4,824,051, entitled “Orbital System Including a Tethered Satellite,” issued Jan. 12, 1987 to Engelking, discloses use of an electrodynamic tether attached to a satellite to compensate for the air drag and the orbit degradation.
U.S. Pat. No. 3,868,072, entitled “Orbital Engine,” issued Feb. 25, 1975, to Fogarty, discloses a tether to rotate/revolve one mass about the other and provide energy.
U.S. Pat. No. 3,582,016, entitled “Satellite Attitude Control Mechanism and Method,” issued Jun. 1, 1971, to Sherman, is a study about transverse waves and rotational dynamics. It does not disclose electrodynamics or use of magnetic fields.
While most of the early estimates of performance of electrodynamic tethers were based on the so-called “static stability” considerations, when non-stationary processes were ignored, in the recent years, more attention is being paid to “dynamic stability” considerations, when complex non-stationary dynamic response to real perturbations is taken into account.
V. V. Beletsky and E. M. Levin in “Dynamics of Space Tether Systems,” Advances in the Astronautical Sciences, v. 83, AAS, 1993, described many modes of inherent instabilities of electrodynamic tethers that are observed even in equatorial circular orbits, and even when dynamic models neglect magnetic field variations along the orbit. They pointed out that it would be virtually impossible to operate electrodynamic tether systems anywhere close to the boundaries of “static stability” because of a very strong, uncontrollable or hardly controllable dynamic instability in these regions. It has been shown in this study that realistic expectations for “safe” electric current levels must be typically lowered by an order of magnitude compared to “static” levels because of dynamic instabilities.
More evidence of rigid dynamic instability constraints was accumulated by others, as more detailed and realistic simulations were performed.
R. P. Hoyt and R. L. Forward in “The Terminator Tether: Autonomous Deorbit of LEO Spacecraft for Space Debris Mitigation,” AIAA 00-0329, 38th Aerospace Sciences Meeting & Exhibit, 10-13 Jan. 2000, Reno, Nev., reported that active control had to be applied to copy with dynamic instabilities. The results of their detailed dynamics simulation results showed actual performance levels much lower than described in the '544 Patent based on “static stability” considerations. This was attributed to the dynamic stability constraints.
J. Corsi and L. Less in “Stability and Control of Electrodynamic Tethers for De-orbiting Applications,” IAF-00-S.6.06, 51st International Astronautical Congress, 2-6 Oct. 2000, Rio de Janeiro, Brazil, showed that realistically for deorbiting with an electrodynamic tether, the electric current in the tether must be periodically switched off to prevent libration buildup and rotation onset, thus substantially decreasing deorbiting efficiency of the electrodynamic tether.
Stability problems have been also reported by D. L. Gallagher, J. Moore, and F. Bagenal in their study of “Electrodynamic Tether Propulsion and Power Generation at Jupiter”, NASA/TP-1998-208475, Marshall SFC, June 1998. At the end of the report, the authors suggested that “a rotating system with two spacecraft connected by a small-to-modest length tether should be investigated” as an alternative configuration for Jovian missions in future research, thus turning to the concept of centrifugal stabilization of an electrodynamic tether described earlier in the “Tethers in Space Handbook”, NASA, 1986.
Traditionally, rotation was perceived as a means of creating artificial gravity onboard space stations and an assist in launching and capturing payloads by momentum-exchange tether transportation systems.
It has been noted recently that rotating momentum-exchange transportation systems can be augmented with an electrodynamic tether reboost. R. P. Hoyt in “Design and Simulation of a Tether Boost Facility for LEO-GTO Transport,” AIAA Paper 2000-3866, 36th Joint Propulsion Conference, Huntsville, Ala., Jul. 17-19 2000, described a rapidly rotating ultra-long tether transportation facility placed in a highly elliptical equatorial orbit. After a momentum-exchange payload launch, the facility is expected to regain its orbital energy by using electrodynamic thrust at perigee passages as the only way to do this without propellant.
Rotation, however, has been considered detrimental to the performance of conventional electrodynamic tether systems. As noticed by R. P. Hoyt and R. L. Forward in “The Terminator Tether: Autonomous Deorbit of LEO Spacecraft for Space Debris Mitigation,” AIAA 00-0329, 38th Aerospace Sciences Meeting & Exhibit, 10-13 Jan. 2000, Reno, Nev., “ . . . electrodynamic forces can become quite significant, and without control over the tether dynamics, the instabilities can result in the tether ‘flipping over’ or even beginning to rotate, with a resultant loss of deorbit efficiency and control over the tether system.” As claimed in the '544 Patent cited earlier, the optimal performance of an electrodynamic tether for deorbiting satellites is achieved in a non-spinning mode, with the tether hanging at a certain fixed angle relative to the local vertical.
The present invention teaches an unobvious fact that spinning (fast rotation) is actually very beneficial for performance and design of electrodynamic tether systems. The spinning is presumed fast compared to the orbital revolution.